U.S. patent application number 15/354275 was filed with the patent office on 2017-05-18 for multiloop interferometers for quantum information processing.
The applicant listed for this patent is Massachusetts Institute of Technology. Invention is credited to Andrew J. Kerman.
Application Number | 20170141286 15/354275 |
Document ID | / |
Family ID | 58691662 |
Filed Date | 2017-05-18 |
United States Patent
Application |
20170141286 |
Kind Code |
A1 |
Kerman; Andrew J. |
May 18, 2017 |
MULTILOOP INTERFEROMETERS FOR QUANTUM INFORMATION PROCESSING
Abstract
Structures and techniques, using superconducting
Josephson-junction based circuits, to directly engineer physical
multiqubit (or "many-qubit") interactions in a non-perturbative
manner. In one embodiment, a system for multiqubit interaction
includes: a multispin coupler including a plurality of loops, each
loop having a pair of Josephson junctions; and a plurality of
qubits each inductively coupled to the multispin coupler.
Inventors: |
Kerman; Andrew J.;
(Arlington, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Massachusetts Institute of Technology |
Cambridge |
MA |
US |
|
|
Family ID: |
58691662 |
Appl. No.: |
15/354275 |
Filed: |
November 17, 2016 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
62256191 |
Nov 17, 2015 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 39/025 20130101;
G06N 10/00 20190101 |
International
Class: |
H01L 39/02 20060101
H01L039/02; G06N 99/00 20060101 G06N099/00 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
[0002] This invention was made with government support under
Contract No. FA8721-05-C-0002 awarded by the U.S. Air Force. The
Government has certain rights in the invention.
Claims
1. A system for multiqubit interaction comprising: a multispin
coupler including a plurality of loops, each loop having a pair of
Josephson junctions; and a plurality of qubits each inductively
coupled to the multispin coupler.
2. The system of claim 1 wherein each loop of the multispin coupler
includes an inductive element coupled between the pair of Josephson
junctions and an energy storage element arranged in parallel with a
first one of the pair of Josephson junctions.
3. The system of claim 1 wherein the multispin coupler further
includes a transformer inductively coupled to each of the plurality
of qubits and to each loop of the multispin coupler.
4. The system of claim 1 wherein each of the plurality qubits
includes a first loop and a second loop, the first and second loop
each including a pair of Josephson junctions, the first loop
further including an inductive element coupled between the pair of
Josephson junctions.
5. The system of claim 1 wherein the plurality of qubits includes
three or more qubits.
6. The system of claim 1 wherein the multispin coupler can be
configured to operate in either an energy mode or a current mode,
wherein in energy mode a total potential energy within the
multispin coupler is proportional to a parity operator over each of
the plurality of qubits, wherein current mode a total current
circulating within the multispin coupler is proportional to the
parity operator over each of the plurality of qubits.
7. The system of claim 6 wherein the multispin coupler can be
configured to operate in either an energy mode or a current mode by
adjusting a magnetic flux through one or more of the loops of the
multispin coupler.
8. A system for multiqubit interaction comprising: a first
multispin coupler; a plurality of second multispin couplers each
inductively coupled to the first multispin coupler, wherein the
first multispin coupler and each of the plurality of second
multispin couplers include a plurality of loops, each loop having a
pair of Josephson junctions; and a plurality of qubits coupled to
each of the second multispin couplers.
9. The system of claim 8 wherein each of the plurality of second
multispin couplers is configured to operate in an energy mode
whereby a total potential energy within each of the plurality of
second multispin couplers is proportional to a parity operator over
each of the plurality of qubits coupled thereto; and wherein the
first multispin coupler is configured to operate in a current mode
whereby a total current circulating within the first multispin
coupler is proportional to a parity operator over each of the
plurality of qubits coupled to each of the plurality of second
multispin couplers.
10. The system of claim 8 wherein each of the plurality of second
multispin couplers is configured to operate in a current mode
whereby a total current circulating within each of the plurality of
second multispin couplers is proportional to a parity operator over
each of the plurality of qubits coupled thereto, and wherein the
first multispin coupler is configured to operate in an energy mode
whereby the total potential energy within the first multispin
coupler is proportional to a parity operator over each of the
plurality of qubits coupled to each of the plurality of second
multispin couplers.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit under 35 U.S.C.
.sctn.119(e) of U.S. Provisional Application No. 62/256,191 filed
Nov. 17, 2015, which application is incorporated herein by
reference in its entirety.
BACKGROUND
[0003] Multiqubit quantum entanglement is a central physical
resource on which the non-classical computational power of quantum
information technology is based. As a result of this, known quantum
information processing methods with the potential to achieve
substantial performance improvement over classical techniques are
built on methods for producing and exploiting large-scale quantum
entanglement. The two most well-known quantum-processing paradigms
are: digital quantum computing, which is expected to provide
exponential performance enhancement most notably for problems in
cryptography (Shor's algorithm) and quantum simulation of chemical
and biological molecules (quantum phase estimation algorithm); and
quantum annealing, where engineered quantum fluctuations may
provide qualitative enhancement in the heuristic sampling of
classical optimization problems.
[0004] In both of these quantum-processing paradigms, the machinery
for construction and maintenance of large-scale quantum
entanglement relies on pairwise interactions between qubits.
Larger-scale entanglement is then built up by combining many of
these pairwise interactions, either by applying them successively
in time in a pulsed manner, or by engineering many static pairwise
interactions simultaneously to obtain an effective higher-order
interaction perturbatively. This is largely limited by physical
hardware constraints: two-qubit interactions are the only
physical-level entangling operations that have yet been
demonstrated in any qubit modality.
SUMMARY
[0005] It is appreciated herein that multiqubit interactions have
engineering potential for improving the performance and scalability
of quantum information processing systems. For example, in the case
of digital quantum computing using topological encoding for
fault-tolerance, the fundamental interactions that are required
involve four qubits. Realizing this with two-qubit interactions
requires four individual two-qubit gates, and introduces an
entirely new class of error processes relative to the idealized
case. Block encoding in these schemes, which promises even higher
performance, requires even higher-order interactions (e.g.,
involving more than four qubits). For quantum annealing, the
realization of large-scale entanglement using two-spin interactions
only (as in some existing commercial quantum information processing
systems) is exponentially inefficient in the number of spins
involved, and this is an important reason that no evidence for
quantum-enhanced performance has yet been observed in these
machines.
[0006] Described herein are structures and techniques, using
superconducting Josephson-junction based circuits, to directly
engineer physical multiqubit (or "many-qubit") interactions in a
non-perturbative manner.
[0007] According to one aspect of the disclosure, a system for
multiqubit interaction includes: a multispin coupler including a
plurality of loops, each loop having a pair of Josephson junctions;
and a plurality of qubits each inductively coupled to the multispin
coupler. In some embodiments, each loop of the multispin coupler
includes an inductive element coupled between the pair of Josephson
junctions and an energy storage element arranged in parallel with a
first one of the pair of Josephson junctions. In certain
embodiments, the multispin coupler further includes a transformer
inductively coupled to each of the plurality of qubits and to each
loop of the multispin coupler. In many embodiments, each of the
plurality qubits includes a first loop and a second loop, the first
and second loop each including a pair of Josephson junctions, the
first loop further including an inductive element coupled between
the pair of Josephson junctions. In some embodiments, the plurality
of qubits includes three or more qubits. In particular embodiments,
the multispin coupler can be configured to operate in either an
energy mode or a current mode, wherein in energy mode a total
potential energy within the multispin coupler is proportional to a
parity operator over each of the plurality of qubits, wherein
current mode a total current circulating within the multispin
coupler is proportional to the parity operator over each of the
plurality of qubits. In many embodiments, the multispin coupler can
be configured to operate in either an energy mode or a current mode
by adjusting a magnetic flux through one or more of the loops of
the multispin coupler.
[0008] According to another aspect of the disclosure, a system for
multiqubit interaction includes: a first multispin coupler; a
plurality of second multispin couplers each inductively coupled to
the first multispin coupler, wherein the first multispin coupler
and each of the plurality of second multispin couplers include a
plurality of loops, each loop having a pair of Josephson junctions;
and a plurality of qubits coupled to each of the second multispin
couplers.
[0009] In some embodiments, each of the plurality of second
multispin couplers is configured to operate in an energy mode
whereby a total potential energy within each of the plurality of
second multispin couplers is proportional to a parity operator over
each of the plurality of qubits coupled thereto, wherein the first
multispin coupler is configured to operate in a current mode
whereby a total current circulating within the first multispin
coupler is proportional to a parity operator over each of the
plurality of qubits coupled to each of the plurality of second
multispin couplers.
[0010] In various embodiments, each of the plurality of second
multispin couplers is configured to operate in a current mode
whereby a total current circulating within each of the plurality of
second multispin couplers is proportional to a parity operator over
each of the plurality of qubits coupled thereto, and wherein the
first multispin coupler is configured to operate in an energy mode
whereby the total potential energy within the first multispin
coupler is proportional to a parity operator over each of the
plurality of qubits coupled to each of the plurality of second
multispin couplers.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The foregoing features may be more fully understood from the
following description of the drawings in which:
[0012] FIG. 1 is a block diagram of a system to provide multiqubit
interactions using a multispin coupler, according to an embodiment
of the disclosure;
[0013] FIG. 2 is a diagram of a circuit to provide multiqubit
interactions using a multiloop superconducting quantum interference
device (mSQUID), according to another embodiment of the
disclosure;
[0014] FIG. 3 is a graph illustrating different modes of mSQUID
operation, according to embodiments of the disclosure; and
[0015] FIG. 4 is a diagram of an inverse paramagnetic tree
structure to provide multiqubit interactions.
[0016] The drawings are not necessarily to scale, or inclusive of
all elements of a system, emphasis instead generally being placed
upon illustrating the concepts, structures, and techniques sought
to be protected herein.
DETAILED DESCRIPTION
[0017] Referring to FIG. 1, according to an embodiment of the
disclosure, a system 100 includes a plurality of qubits 102a-102n
(generally denoted 102) and a multispin coupler 104.
[0018] In many embodiments, the qubits 102 are provided a flux
qubits. As is known, flux qubits (also known as persistent current
qubits) are micrometer-sized loops of superconducting metal
interrupted by a number of Josephson junctions. The Josephson
junction parameters may be fabricated such that a persistent
current will flow continuously when an external flux is applied.
The computational basis states of the qubit are defined by the
circulating currents, which can flow either clockwise or
counter-clockwise.
[0019] The multispin coupler 104 provides multiqubit (i.e., two or
more) interactions in a non-perturbative manner. In some
embodiments, the multispin coupler 104 may be provided as a
multi-loop circuit.
[0020] Referring to the embodiment of FIG. 2, a circuit 200 can
provide multiqubit interactions using a multispin coupler. The
circuit 200 includes a plurality of qubits 202a-202n (202
generally) and a multispin coupler 204 provided as a multiloop DC
interferometer (referred to herein as an "mSQUID"). The mSQUID 204
includes a transformer 206 to which each of the qubits 202 may be
inductively coupled.
[0021] In the embodiment of FIG. 2, the qubits 202 are provided as
flux qubits each having two pairs of Josephson junctions (in FIG.
2, individual Josephson junctions are denoted "X"). For example,
qubit 202a is denoted as having a first pair of Josephson junctions
210 arranged about a first loop 212, and a second pair of Josephson
junctions 214 arranged about a second loop 216.
[0022] Each qubit 202 further includes an inductive element 215 to
provide inductive coupling with the mSQUID transformer 206. As used
herein, the term "inductive element" refers to any circuit element
that stores energy in a magnetic field, including linear and
nonlinear inductors. In the embodiment of FIG. 2, the qubit
inductive element is coupled between the pair of Josephson
junctions 210 within the first loop 212.
[0023] In other embodiments, the qubit inductive element 215 could
be placed within the second loop 216 (e.g., between Josephson
junctions 214). In certain embodiments, a qubit may include
multiple indicative elements (e.g. one in each of its two loops).
Those skilled in the art will understand that each loop of a qubit
has a finite self-inductance to which other loops can couple via a
mutual inductance. By coupling to a loop with the larger junctions
(e.g., loop 212 in FIG. 2), this is effectively coupling to a
operator, while the loop with the smaller junctions (e.g., loop 216
in FIG. 2) is effectively coupling to an X operator.
[0024] It should be understood that the concepts and structures
sought to be protected herein are not limited to the qubit
structure shown in FIG. 2. For example, in some embodiments, any
qubit which has a state-dependent magnetic moment can be used. The
illustrative mSQUID 204 includes the transformer 206, a multiloop
circuit 208 inductively coupled thereto, and an output 228. The
transformer 206 may include a Josephson junction pair 218, as
shown. Including Josephson junctions in the transformer can allow
flux to enter and leave the transformer loop when it is cooling (so
that flux is not trapped, as would happen in a closed
superconducting loop). Josephson junction pair 218 may also allow
the transformer 206 to be tunable.
[0025] The multiloop circuit 208 includes a plurality of loops
220a-220n (220 generally), each having a Josephson junction pair
222, an inductive element (e.g., a linear or non linear inductive
element) 224 to provide inductive coupling to the transformer 206,
and an energy storage element 226 (e.g., a capacitor) arranged in
parallel with one of the loop Josephson junctions. To promote
clarity in the figure, the Josephson junction pair, inductive
element, and energy storage element are labeled only for a first
loop 220a.
[0026] It should be understood that the number of loops 220 within
the circuit 208 may be selected based on requirements for a given
application. In particular, the number of loops 220 determines the
number of free parameters that can be used to define the shape of
mSQUID's nonlinear energy vs. flux characteristic. Thus, increasing
the number of loops 200 may provide additional control required for
a given application. However, it will be appreciated that
increasing the number of loops 200 may require more flux from the
transformer 206 for the mSQUID to function.
[0027] It will be appreciated that the design of each mSQUID loop
220 is based on that of a conventional DC SQUID, hence the name
mSQUID (or multiloop SQUID). As is known, the effective Josephson
potential energy of a DC SQUID can be modulated by changing the
relative gauge-invariant phase between the two Josephson junctions,
adjusted via the magnetic flux through the respective loop. In the
case of a multiloop SQUID 204, an arbitrary potential energy
function vs. uniform flux can be realized by adjusting the
individual flux bias through the multispin coupler loops 220. In
particular, the constant phase offset between each Josephson
junction can be controlled to realize arbitrary potential energy
function vs. uniform flux. The result is that for flux .PHI.
coupled to a common transformer and applied equally to all loops
220 of the mSQUID 204, an arbitrary potential energy function
U(.PHI.) can be engineered by appropriately tuning the offset
fluxes applied to the individual loops 222.
[0028] In operation, each qubit 202 nominally produces a
state-dependent flux.+-..PHI..sub.q through the transformer 206.
For N qubits 202 coupled to transformer 206, the total
qubit-state-dependent flux (i.e., the total state-dependent flux
from all qubits, which is the input to the mSQUID) then takes one
of the N+1 different values: .PHI..sub.k.epsilon..PHI..sub.q {-N,
-N+2 . . . 0 . . . N-2, N}.
[0029] It is appreciated herein the mSQUID 204 can be dynamically
adjusted such that it behaves like a current source:
(.PHI.)=I.PHI., an inductor: U(.PHI.)=.PHI..sup.2/2L, or any more
complicated nonlinear magnetic element whose behavior can be
expressed as a potential energy U(.PHI.) (the classical parameters
I, L, etc. are related to the Taylor expansion coefficients of the
energy vs. flux characteristic about a chosen bias point).
Furthermore, since the internal Josephson frequencies of the
multispin coupler 204 can be kept relatively large (>100 GHz),
this electrical behavior can be preserved over an extremely wide
frequency range. As a result, the mSQUID 204 may be used in
parametric nonlinear quantum devices--such as amplifiers and
frequency converters--that conventionally rely on the "bare"
Josephson nonlinearity, which is relatively weak by comparison.
[0030] Referring to FIG. 3, in various embodiments, an mSQUID
(e.g., mSQUID 204 in FIG. 2) can be operated in two different
modes: energy mode and current mode. In energy mode, the potential
energy within the mSQUID's multiloop circuit is proportional to the
N-qubit parity operator {circumflex over
(P)}.sub.N.ident..PI..sub.i=1.sup.N{circumflex over
(.sigma.)}.sub.i.sup.z such that: (.PHI..sub.k)=.DELTA.E{circumflex
over (P)}.sub.N, where {circumflex over (.sigma.)}.sub.i.sup.z is
the Z operator for the ith qubit. In current mode, the total
current circulating within the mSQUID's multiloop circuit is
proportional to the N-qubit parity operator {circumflex over
(P)}.sub.N .ident..PI..sub.i=1.sup.N{circumflex over
(.sigma.)}.sub.i.sup.z such that I(.PHI..sub.k)=I.sub.m{circumflex
over (P)}.sub.N. In some embodiments, the mSQUID 204 can be
configured to operate in a given mode by adjusting the magnetic
flux through the mSQUID loops 220, as discussed above.
[0031] The graph 300 in FIG. 3 illustrates these two modes of
mSQUID operation. The top curve 302 illustrates energy mode,
wherein the total potential energy as a function of possible qubit
flux values (denoted by circles along the curve 302, e.g., circle
304) is effectively proportional to a multispin (in this case,
three-spin) operator: =.DELTA.E{circumflex over
(.sigma.)}.sub.1.sup.z{circumflex over
(.sigma.)}.sub.2.sup.z{circumflex over (.sigma.)}.sub.3.sup.z. In
energy mode, the potential energy vs. input flux can be tailored
such that at the flux points accessible via the qubits' input to
the transformer, the mSQUID energy takes on one of two values,
according to the total parity of the qubit state. This results in
an effective interaction between the qubits.
[0032] The bottom curve 306 illustrates the current mode of
operation, in which the potential energy is the same at all of the
possible qubit flux values (denoted by circles along the curve 306,
e.g., circle 308), but the mSQUID circulating current (proportional
to the slope of the energy vs. flux) is proportional to a
multiqubit operator: I=I{circumflex over
(.sigma.)}.sub.1.sup.z{circumflex over
(.sigma.)}.sub.2.sup.z{circumflex over (.sigma.)}.sub.3.sup.z. In
current mode, the potential energy is kept constant and independent
of the qubits' input state, while the effective mSQUID circulating
current (which is given by the derivative of the energy with
respect to the flux) assumes only two values, and is proportional
to the total qubit state parity.
[0033] It should be understood that although FIG. 3 illustrates
mSQUID operation for the case of three (3) qubits, the concepts,
structures, and techniques sought to be protected herein be used to
effect interactions between arbitrary numbers of qubits (e.g.,
between N qubits, where N>1).
[0034] FIG. 4 illustrates how the different modes of mSQUID
operation can be combined using a so-called "inverse paramagnetic
tree" structure to provide higher-order multiqubit interactions. An
inverse paramagnetic tree structure 400 may include a root mSQUID
406, a plurality of intermediate mSQUIDs (e.g., intermediate
mSQUIDs 404a) coupled thereto, and a plurality of qubits (e.g.,
qubit 402a) coupled to each of the intermediate mSQUIDs. In the
embodiment of FIG. 4, the structure 400 includes three (3)
intermediate mSQUIDs 404a-404c, each having a group of three (3)
qubits (or "spins") 402 coupled thereto. In particular, qubits
402a-402c are coupled to a first intermediate mSQUID 404a, qubits
402d-402f are coupled to a second intermediate mSQUID 404b, and
qubits 402g-402i are coupled to a third intermediate mSQUID
404c.
[0035] The root mSQUID 406 may be operated in a different mode than
the intermediate mSQUIDs 404. For example, each group of M (e.g.,
M=3) qubits 402 may be coupled to a respective intermediate mSQUID
404 operating in current mode, such that the total circulating
current in each intermediate mSQUID 404 is proportion to a M-qubit
operator product, with M=3 in FIG. 4. In turn, each intermediate
mSQUID 404 may be coupled to a root mSQUID 406 operating in energy
mode, such that the total energy in the root mSQUID 406 is
proportional to an N-qubit operator product, with N=9 in FIG. 4.
Thus, it will be appreciated that the parities of multiple groups
of M qubits 402 can be combined, resulting in an effective
interaction between all N qubits.
[0036] It should be understood that the tree structure 400 shown in
FIG. 4 is merely one example and that an inverse paramagnetic tree
structure 400 could include additional levels (i.e., more than one
level of intermediate mSQUIDs 404) and could have more than three
(3) connections at each levels of the tree. Moreover, the number of
connections at each level may vary in some embodiments. For
example, the root mSQUID could be coupled to a certain number of
intermediate mSQUIDs, each of which could be coupled to a different
number of qubits. Further, the structure 400 can be used to realize
either X or Z operators by connecting to either the Z or X loops of
the qubits 402.
[0037] It is appreciated that the concepts, structures, and
techniques described herein may be used to provide high-order qubit
interactions and, as such, could have transformational importance
in both quantum annealing and in digital quantum information
processing.
[0038] All references cited herein are hereby incorporated herein
by reference in their entirety.
[0039] Having described certain embodiments, which serve to
illustrate various concepts, structures, and techniques sought to
be protected herein, it will be apparent to those of ordinary skill
in the art that other embodiments incorporating these concepts,
structures, and techniques may be used. Elements of different
embodiments described hereinabove may be combined to form other
embodiments not specifically set forth above and, further, elements
described in the context of a single embodiment may be provided
separately or in any suitable sub-combination. Accordingly, it is
submitted that the scope of protection sought herein should not be
limited to the described embodiments but rather should be limited
only by the spirit and scope of the following claims.
* * * * *